Photoinactivation of catalase in vitro and in leaves

Purified catalase from bovine liver and catalase of isolated intact peroxisomes from rye leaves were inactivated in vitro by irradiation with visible light. During photoinactivation the protein moiety of pure catalase was not cleaved; however, the electrophoretic mobility of the native enzyme was decreased, and a major portion of enzyme-bound heme was dissociated. In a suspension of isolated chloroplasts photoinactivation of pure or peroxisomal catalase was mediated by light absorption in the chloroplasts. Both the direct and the chloroplast-mediated photoinactivation of catalase were affected little by the presence of D2O or superoxide dismutase but were greatly retarded by formate. In isolated peroxisomes substantial photoinactivation of catalase occurred only in the presence of nonphotosynthesizing but not in the presence of photosynthesizing isolated chloroplasts. Substantial and selective photoinactivation of catalase was also observed in vivo when leaf sections from various plant species (rye, pea, sunflower, cucumber, maize) were irradiated with light of high intensity in the presence of the translation inhibitors cycloheximide or 2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide, while catalase activity was much less or not affected in 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated or untreated control sections. The extent of photoinactivation of catalase in leaves depended on light intensity and also occurred in red light. The results suggest that photoinactivation of catalase generally occurs in leaves under high light intensity, though it is not apparent under normal physiological conditions because it is compensated for by new synthesis. Apparent photoinactivation of catalase has to be regarded as an early indication of photodamage in leaves and conceivably enhances its progress.     

Developmental studies on microbodies in wheat leaves : I. Conditions influencing enzyme development

Catalase, glycolate oxidase, and hydroxypyruvate reductase, enzymes which are located in the microbodies of leaves, show different developmental patterns in the shoots of wheat seedlings. Catalase and hydroxypyruvate reductase are already present in the shoots of ungerminated seeds. Glycolate oxidase appears later. All three enzymes develop in the dark, but glycolate oxidase and hydroxypyruvate reductase have only low activities. On exposure of the seedlings to continuous white light (14.8 × 10³ ergs cm⁻² sec⁻¹), the activity of catalase is doubled, and glycolate oxidase and hydroxypyruvate reductase activities increase by 4- to 7-fold. Under a higher light intensity, the activities of all three enzymes are considerably further increased. The activities of other enzymes (cytochrome oxidase, fumarase, glucose-6-phosphate dehydrogenase) are unchanged or only slightly influenced by light. After transfer of etiolated seedlings to white light, the induced increase of total catalase activity shows a much longer lag-phase than that of glycolate oxidase and hydroxypyruvate reductase. It is concluded that the light-induced increases of the microbody enzymes are due to enzyme synthesis. The light effect on the microbody enzymes is independent of chlorophyll formation or the concomitant development of functional chloroplasts. Short repeated light exposures which do not lead to greening are very effective. High activities of glycolate oxidase and hydroxypyruvate reductase develop in the presence of 3-amino-1,2,4-triazole which blocks chloroplast development. The effect of light is not exerted through induced glycolate formation and appears instead to be photomorphogenetic in character.     

Mode of interference of chlorosis-inducing herbicides with peroxisomal enzyme activities

In rye leaves ( Secale cereale L. cv. Petkus "Kustro") bleached in the presence of the chlorosis-inducing herbicides aminotriazole, haloxidine, San 6706 or difunone in white light of 54.2 W m^-2 (5000 lx), catalase activity was very low. In addition, the activities of glycolate oxidase and hydroxypyruvate reductase were strongly diminished in treatments with San 6706 and difunone. The lowering of the peroxisomal enzyme activities was observed in red, but not in blue light and did not occur after treatment with the non-bleaching pyridazinone derivative San 9785. The deficiencies of the peroxisomal enzymes did not appear to be involved in the initiation of the chlorosis. Instead they are probably produced as secondary consequences of the bleaching. Low peroxisomal enzyme activities were also obtained without herbicide treatment by growing the leaves in an atmosphere of 2% O₂ and 3% CO₂, but in this case were not accompanied by an increased sensitivity of the Chl to photooxidative bleaching. The peroxisomal enzymes reached as high activities as in untreated controls when the herbicide-treated leaves were grown at a low light intensity of 0.106 W m^-2 (10 lx). After transfer of herbicide-treated leaves grown under 0.106 W m^-2 to 306 W m^-2 (30 000 lx), catalase was strongly inactivated, even at 0°C. In treatments with San 6706 and difunone the increase of the activities of glycolate oxidase and hydroxypyruvate reductase was either stopped, remaining unchanged, or the enzymes were slightly inactivated after exposure to 306 W m^-2 (30 000 lx). The observations suggest that the inactivation of peroxisomal enzymes results from photooxidative events in the chloroplasts.     

光和底物对乙醇酸氧化酶的基因表达的诱导

从菠菜中提纯了乙醇酸氧化酶并制备其抗体,经免疫双扩散、Ｗｅｓｔｅｒｎｂｌｏｔ和Ｎｏｒｔｈｅｒｎｂｌｏｔ证实水稻和豌豆黄化苗中不存在乙醇酸氧化酶。在黑暗中,底物可促进该酶基因的表达,而在黄化苗光照初期,推测光可能是不经过底物促进该酶基因的表达。     

Glycolate oxidase content of microbodies as affected by nitrate

Glycolate oxidase is loosely held by microbodies obtained from etiolated barley (Hordeum vulgare L.) leaves depleted of nitrate. Defined centrifugation conditions cause the complete detachment of the enzyme from the microbodies. Addition of nitrate to these plants brings about a greater retention of glycolate oxidase by the microbodies. Synthesis of a nitrate-induced protein seems to be responsible for the enhanced retention of glycolate oxidase. Catalase, on the contrary, is strongly attached to the microbodies under all nutritional and experimental conditions considered.     

菜心乙醇酸氧化酶的纯化和催化特性分析

用改进后的方法,从菜心绿叶中分离纯化得到一个亚基分子量为42kD的乙醇酸氧化酶,用氧电极法测定该酶同时能催化乙醇酸和乙醛酸的氧化。     

Resistance to photoinhibition of photosystem II and catalase and antioxidative protection in high mountain plants

In leaves of three alpine high mountain plants, Homogyne alpina, Ranunculus glacialis and Soldanella alpina, both photosystem II (PSII) and the enzyme catalase appeared to he highly resistant to photoinactivation under natural field conditions. While the Dl protein of PSII and catalase have a rapid turnover in light and require continuous new protein synthesis in non-adapted plants, little apparent photoinactivation of PSII or catalase was induced in the alpine plants by translation inhibitors or at low temperature, suggesting that turnover of the Dl protein and catalase was slow in these leaves. In vitro PSII was rapidly inactivated in light in isolated thylakoids from H. alpina and R. glacialis. In isolated intact chloroplasts from R. glacialis, photoinactivation of PSII was slower than in thylakoids. Partially purified catalase from R. glacialis and S. alpina was as sensitive to photoinactivation in vitro as catalases from other sources. Catalase from H. alpina had, however, a 10-fold higher stability in light. The levels of xanthophyll cycle carotenoids, of the antioxidants ascorbate and glulathione, and of the activities of catalase, superoxide dismutase and glutathione reductase were very high in S. alpina, intermediate in H. alpina, but very low in R. glacialis. However, isolated chloroplasts from all three alpine species contained much higher concentrations of ascorbate and glutathione than chloroplasts from lowland plants.     

Photoinactivation of Catalase Occurs under Both High- and Low-Temperature Stress Conditions and Accompanies Photoinhibition of Photosystem II

Severe photoinactivation of catalase (EC 1.11.1.6) and a decline of variable fluorescence (F_v), indicating photoinhibition of photosynthesis, were observed as rapid and specific symptoms in leaves exposed to a high heat-shock temperature of 40°C as well as in leaves exposed to low chilling temperatures in white light of only moderately high photosynthetic photon flux density of 520 μE m⁻² s⁻¹. Other parameters, such as peroxidase (EC 1.11.1.7), glycolate oxidase (EC 1.1.3.1), glutathione reductase (EC 1.6.4.2), or the chlorophyll content, were hardly affected under these conditions. At a compatible temperature of 22°C, the applied light intensity did not induce severe photoinactivations. In darkness, exposures to high or low temperatures did not affect catalase levels. Also, decline of F_v in light was not related to temperature sensitivity in darkness. The effective low-temperature ranges inducing photoinactivation of catalase differed significantly for chilling-tolerant and chilling-sensitive plants. In leaves of rye (Secale cereale L.) and pea (Pisum sativum L.), photoinactivation occurred only below 15°C, whereas inactivation occurred at 15°C in cucumber (Cucumis sativus L.) and maize (Zea mays L.). The behavior of F_v was similar, but the difference between chilling-sensitive and chilling-tolerant plants was less striking. Whereas the catalase polypeptide, although photoinactivated, was not cleaved at 0 to 4°C, the D1 protein of photosystem II was greatly degraded during the low-temperature treatment of rye leaves in light. Rye leaves did not exhibit symptoms of any major general photodamage, even when they were totally depleted of catalase after photoinactivation at 0 to 4°C, and catalase recovered rapidly at normal temperature. In cucumber leaves, the decline of catalase after exposures to bright light at 0 to 4°C was accompanied by bleaching of chlorophyll, and the recovery observed at 25°C was slow and required several days. Similar to the D1 protein of photosystem II, catalase differs greatly from other proteins by its inactivation and high turnover in light. Inasmuch as catalase and D1 protein levels depend on continuous repair synthesis, preferential and rapid declines are generally to be expected in light whenever translation is suppressed by stress actions, such as heat or chilling, and recovery will reflect the repair capacity of the plants.     

Photoinactivation of the detoxifying enzyme nitrophenol reductase from Rhodobacter capsulatus

The phototrophic bacterium Rhodobacter capsulatus E1F1 detoxifies 2,4-dinitrophenol by inducing an NAD(P)H-dependent iron flavoprotein that reduces this compound to the less toxic end product 2-amino-4-nitrophenol. This nitrophenol reductase was stable in crude extracts containing carotenes, but it became rapidly inactivated when purified protein was exposed to intense white light or moderate blue light intensities, especially in the presence of exogenous flavins. Red light irradiation had no effect on nitrophenol reductase activity. Photoinactivation of the enzyme was irreversible and increased under anoxic conditions. This photoinactivation was prevented by reductants such as NAD(P)H and EDTA and by the excited flavin quencher iodide. Addition of superoxide dismutase, catalase, tryptophan or histidine did not affect photoinactivation of nitrophenol reductase, thus excluding these reactive dioxygen species as the inactivating agent. Substantial protection by 2,4-dinitrophenol also took place when the enzyme was irradiated at a wavelength coinciding with one of the absorption peaks of this compound (365nm). These results suggest that the lability of nitrophenol reductase was due to the absorption of blue light by the flavin prosthetic group, thus producing an excited flavin that might irreversibly oxidize some functional group(s) necessary for enzyme catalysis. Nitrophenol reductase may be preserved in vivo from blue light photoinactivation by the high content of carotenes and excess of reducing equivalents in phototrophic growing cells.Abbreviations 2,4-DNP 2,4-dinitrophenol - ANP 2-amino-4-nitrophenol - EDTA ethylenediamine tetraacetic acid - MES 2-(N-Morpholino) ethanesulfonic acid - NPR nitrophenol reductase     

Molecular structure and subcellular localization of spinach leaf glycolate oxidase

Glycolate oxidase (E.C. 1.1.3.1) was purified from spinach leaves (Spinacia oleracea). The molecular weight of the native protein was determined by sucrose density gradient centrifugation to be 290,000 daltons (13S), whereas that of the monomeric form was 37,000 daltons. The quaternary structure of the holoenzyme is likely to be octameric, analogous to pumpkin cotyledon glycolate oxidase [Nishimura et al, 1982]. The subcellular localization of the enzyme was studied using linear sucrose density gradient centrifugation, and it was found that glycolate oxidase activity is detectable in both leaf peroxisomal and supernatant fractions, but not in chloroplasts and mitochondria; the activity distribution pattern is essentially similar to that for catalase, a known leaf peroxisomal enzyme. Ouchterlony double diffusion and immunotitration analyses, demontrated that the rabbit antiserum against purified spinach leaf glycolate oxidase cross-reacted, identically, with the enzyme molecules present in two different subcellular fractions, i.e, the leaf peroxisome and supernatant fractions. It is thus concluded that the enzyme present in the supernatant is due to the disruption of leaf peroxisomes during the isolation, and hence glycolate oxidase is exclusively localized in leaf peroxisomes in spinach leaves.     

Molecular structure and subcellular localization of spinach leaf glycolate oxidase

Glycolate oxidase (E.C. 1.1.3.1) was purified from spinach leaves (Spinacia oleracea). The molecular weight of the native protein was determined by sucrose density gradient centrifugation to be 290,000 daltons (13S), whereas that of the monomeric form was 37,000 daltons. The quaternary structure of the holoenzyme is likely to be octameric, analogous to pumpkin cotyledon glycolate oxidase [Nishimura et al, 1982]. The subcellular localization of the enzyme was studied using linear sucrose density gradient centrifugation, and it was found that glycolate oxidase activity is detectable in both leaf peroxisomal and supernatant fractions, but not in chloroplasts and mitochondria; the activity distribution pattern is essentially similar to that for catalase, a known leaf peroxisomal enzyme. Ouchterlony double diffusion and immunotitration analyses, demonstrated that the rabbit antiserum against purified spinach leaf glycolate oxidase cross-reacted, identically, with the enzyme molecules present in two different subcellular fractions, i.e, the leaf peroxisome and supernatant fractions. It is thus concluded that the enzyme present in the supernatant is due to the disruption of leaf peroxisomes during the isolation, and hence glycolate oxidase is exclusively localized in leaf peroxisomes in spinach leaves.     

Characterization of a cDNA Encoding Lens culinaris Glycolate Oxidase and Developmental Expression of Glycolate Oxidase mRNA in Cotyledons and Leaves

mRNA obtained from green leaves of lentil (Lens culinaris) was used to construct a cDNA library in phage λgt11. The cDNA library was screened with antibodies raised against lentil glycolate oxidase and catalase. Clone CL 1 containing the full-length sequence complementary to glycolate oxidase mRNA was characterized and sequenced. In addition, a 800-base pair catalase cDNA clone was sequenced. To prove the correlation of cDNA insert in CL 1 with glycolate oxidase, the cDNA was transcribed in vitro. The mRNA was translated in vitro yielding a 43 kilodalton protein immunoprecipitable with anti-glycolate oxidase serum. Nucleotide sequences of lentil cDNA and spinach cDNA were 86% identical. Lentil glycolate oxidase was characterized by a C-terminal sequence -P-R-A-L-P-R-L. The expression of glycolate oxidase mRNA in cotyledons, leaves and roots was compared with that of catalase. In leaves, the relative amount of glycolate oxidase mRNA increased during the first 2 days of greening, but decreased later, and was hardly detectable during senescence. In cotyledons of germinating seeds, the level of glycolate oxidase mRNA was markedly lower than the catalase mRNA.     

Glycolate oxidase isoforms are distributed between the bundle sheath and mesophyll tissues of maize leaves

Glycolate oxidase (EC 1.1.3.15) activity was detected both in the bundle sheath (79%) and mesophyll (21%) tissues of maize leaves. Three peaks of glycolate oxidase activity were separated from maize leaves by the linear KCl gradient elution from the DEAE-Toyopearl column. The first peak corresponded to the glycolate oxidase isoenzyme located in the bundle sheath cells, the second peak had a dual location and the third peak was related to the mesophyll fraction. The mesophyll isoenzyme showed higher affinity for glycolate (Km 23 micromol x L(-1)) and a higher pH optimum (7.5-7.6) as compared to the bundle sheath isoenzyme (Km 65 micromol x L(-1), pH optimum 7.3). The bundle sheath isoenzyme was strongly activated by isocitrate and by succinate while the mesophyll isoenzyme was activated by isocitrate only slightly and was inhibited by succinate. It is concluded that although the glycolate oxidase activity is mainly attributed to the bundle sheath, conversion of glycolate to glyoxylate occurs also in the mesophyll tissue of C4 plant leaves.     

Diffusion of extracellular hydrogen peroxide into intracellular compartments of human neutrophils. Studies utilizing the inactivation of myeloperoxidase by hydrogen peroxide and azide

It is well known that catalase is transformed to nitric oxide-Fe2+-catalase by hydrogen peroxide (H2O2) plus azide. In this report, we show that myeloperoxidase is also inactivated by H2O2 plus azide. Utilizing this system, we studied the presence and source of intracellular H2O2 generated by activated neutrophils. Stimulation of neutrophils with phorbol myristate acetate (PMA, 100 ng/ml) plus azide (5 mM) for 30 min completely inactivated intragranular myeloperoxidase and reduced cytosolic catalase to 35% of resting cells. This intracellular inactivation of heme enzymes did not occur in normal neutrophils incubated with either PMA or azide alone or in neutrophils from patients with chronic granulomatous disease (CDG) which cannot produce H2O2 in response to PMA. Incubation of neutrophils with azide and a H2O2 generating system (glucose-glucose oxidase) inactivated 41% of neutrophil myeloperoxidase. Glutathione-glutathione peroxidase (GSH-GSH peroxidase), an extracellular H2O2 scavenger, totally protected neutrophil myeloperoxidase from inactivation by azide plus glucose-glucose oxidase. In addition, when a mixture of normal and CGD cells was stimulated with PMA in the presence of azide, 90% of the myeloperoxidase in CGD neutrophils was inactivated. Therefore, H2O2 released extracellularly from activated neutrophils can diffuse into cells. In contrast, myeloperoxidase in normal polymorphonuclear leukocytes stimulated with PMA in the presence of azide and GSH-GSH peroxidase was 75% inactivated. Thus, the results indicate that a GSH-GSH peroxidase-insensitive pool of H2O2 is also generated, presumably at the plasma membrane, and this pool of H2O2 can undergo direct internal diffusion to inactivate myeloperoxidase.     

14CO2 fixation and glycolate metabolism in the dark in isolated maize (Zea mays L.) bundle sheath strands

Bundle sheath strands capable of assimilating up to 68 μmoles CO₂ per mg chlorophyll per hr in the dark have been isolated from fully expanded leaves of Zea mays L. This dark CO₂-fixing system is dependent on exogenous ribose-5-phosphate, ADP or ATP, and Mg²⁺ for maximum activity. The principal product of dark fixation in this system is 3-phosphoglycerate, indicating that the CO₂-fixing reaction is mediated by ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39). The rate of dark CO₂ uptake in the strands in the presence of saturating levels of ribose-5-phosphate plus ADP is inhibited by oxygen. The inhibitory effect of oxygen is rapidly and completely reversible, and is relieved by increased levels of CO₂. Glycolate is synthesized in this dark system in the presence of [U-¹⁴C]ribose-5-phosphate, ADP, oxygen, and an inhibitor of glycolate oxidase (EC 1.1.3.1). Glycolate formation is completely abolished by heating the strands, and the rate of glycolate synthesis is markedly reduced by either lowering the oxygen tension or increasing the level of CO₂.These results, obtained with intact cells in the absence of light, indicate that the direct inhibitory effect of oxygen on photosynthesis is associated with photosynthetic carbon metabolism, probably at the level of ribulose-1,5-bisphosphate carboxylase, and not with photophosphorylation or photosynthetic electron transport. Furthermore, the findings indicate that the synthesis of glycolate from exogenous substrate can readily occur in the absence of photosynthetic electron transport, an observation consistent with the ribulose-1, 5-bisphosphate “oxygenase” scheme for glycolate formation during photosynthesis.     

植物中草酸积累与光呼吸乙醇酸代谢的关系

对几种C3 和C4 植物中草酸含量及相应的乙醇酸氧化酶活性测定结果表明 :叶片光呼吸强度及其关键酶活性大小与草酸积累量没有相关性 ;植物根中均能积累草酸 ,但未测出乙醇酸氧化酶活性。烟草根、叶中的草酸含量在不同生长时期差异明显 ,且二者呈极显著正相关 (y =2 .5 6 5lnx 2 .137,r =0 .749,P <0 .0 0 1) ,说明根中草酸可能来自叶片。氧化乙醇酸的酶的活性与氧化乙醛酸的酶的活性呈极显著线性正相关 (y =0 .2 41x 0 .0 0 6 ,r=0 .96 7,P <0 .0 0 0 1) ,进一步证实是乙醇酸氧化酶催化了两种底物的反应。烟草在不同生长期叶片中草酸总含量变化与相应的乙醇酸氧化酶活性变化亦没有相关性 ;低磷胁迫可显著诱导烟草根叶中的草酸形成和分泌 ,但并未影响乙醇酸氧化酶活性 ,进一步证明草酸积累与该酶活性大小无关     

Cytochrome P450 2E1 metabolically activates propargyl alcohol: propiolaldehyde-induced hepatocyte cytotoxicity

Pargyline, an antihypertensive agent and monoamine oxidase inhibitor, induces hepatic GSH depletion and hepatotoxicity in vivo in rats [E.G. De Master, H.W. Sumner, E. Kaplan, F. N. Shirota, H.T. Nagasawa, Toxicol. Appl. Pharmacol. 65 (1982) 390-401]. Propargyl alcohol (2-propyn-1-ol), because of its structural similarity to allyl alcohol, was thought to be activated by alcohol dehydrogenase. However, it is a poor substrate compared to allyl alcohol and it was therefore proposed that propargyl alcohol-induced liver injury involved metabolic activation by catalase/H(2)O(2) [E.G. De Master, T. Dahlseid, B. Redfern, Chem. Res. Toxicol. 7 (1994) 414-419]. In the following we showed that; (1) propargyl alcohol-induced cytotoxicity was markedly enhanced in CYP 2E1-induced hepatocytes and prevented by various CYP 2E1 inhibitors but was only slightly affected when alcohol dehydrogenase was inhibited with methylpyrazole/DMSO or when catalase was inactivated with azide or aminotriazole, (2) hepatocyte GSH depletion preceded cytotoxicity and was inhibited by cytochrome P450 inhibitors but not by catalase/alcohol dehydrogenase inhibitors. GSH conjugate formation during propargyl alcohol metabolism by microsomal mixed function oxidase in the presence of GSH was also prevented by anti-rat CYP 2E1 or CYP 2E1 inhibitors, (3) cytotoxicity was prevented when lipid peroxidation was inhibited with antioxidants, desferoxamine (ferric chelator) or dithiothreitol. Propargyl alcohol-induced cytotoxicity and reactive oxygen species formation were markedly increased in GSH-depleted hepatocytes. All of this evidence suggests that propargyl alcohol-induced cytotoxicity involves metabolic activation by CYP 2E1 to form propiolaldehyde that causes hepatocyte lysis as a result of GSH depletion and lipid peroxidation.     

Dependence of catalase photoinactivation in rye leaves on light intensity and quality and characterization of a chloroplast-mediated inactivation in red light

In green or etiolated rye leaves catalase was most efficiently inactivated by blue light absorbed by its prosthetic heme. Red light was ineffective at low intensity but induced marked inactivation in green leaves at higher photon flux, while far-red light was ineffective. At identical intensities of photosynthetically active radiation, Photosystem II (PS II) was equally inactivated by both blue and red light. Since catalase was insensitive to red light and no sensitizer for red light was detected in isolated peroxisomes, the inactivation of catalase observed in leaves in red light must result from photooxidative reactions initiated in the chloroplasts. In a simplified model system the inactivation of isolated catalase was induced by the presence of a suspension of either intact or broken chloroplasts in red light. This chloroplast-mediated inactivation of catalase in vitro was O₂-dependent. It was greatly retarded at low temperature, fully suppressed by the radic al scavenger Trolox, partially retarded by superoxide dismutase, but only little diminished by the singlet oxygen quencher histidine and not affected by dimethylsulfoxide, a hydroxyl radical scavenger. Chloroplast-mediated catalase inactivation in vitro was suppressed by suitable electron acceptors, in particular by methyl viologen. A comparison of the effects of inhibitors, donors, or acceptors for specific sites of the photosynthetic electron transport indicated that an overreduction of PS II and plastoquinone represented the major sources for the formation of O₂ and some unidentified radical that appeared to mediate the inactivation of catalase outside of the chloroplasts. Chloroplast-mediated catalase inactivation provides a means for the detection of a redox signalling system of chloroplasts that was postulated to indicate overreduction of plastoquinones. Similarly as in the in vitro system, catalase inactivation in red light was also in leaves temperature-dependent and stimulated by DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone). These results provide strong evidence that inactivation of catalase initiated by chloroplastic reactions in red light occurred also in leaves under identical conditions as in the model system in vitro.     

Preferential photoinactivation of catalase and photoinhibition of photosystem II are common early symptoms under various osmotic and chemical stress conditions

Activity of catalase (EC 1.11.1.6) and variable fluorescence (F) were measured in sections of rye leaves (Secale cereale L. cv. Halo) that were exposed for 24 h to moderately high irradiance under osmotic or chemical stress conditions (paraquat, DCMU, mannitol, NaCl, CdCl₂, CuSO₄, Pb(NO₃)₂, KNO₂, or K₂SO₃). Changes of the chlorophyll content and of enzyme activities related to peroxide metabolism, such as glycolate oxidase, glutathione reductase, and peroxidase, were assayed for comparison. In the presence of the herbicides paraquat and low DCMU concentrations that exert only partial inhibition of photosynthesis, as well as after most treatments with osmotic or chemical stress factors, catalase markedly declined due to a preferential photoinactivation. At higher DCMU levels catalase did not decline. At low KNO₂ concentrations catalase activity was preferentially increased. In general, photoinactivation of catalase was accompanied by a decline of the F/F_m ratio, indicating photoinhibition of photosystem II, while other parameters were much more stable. Inasmuch as both catalase and the D1 reaction center protein of photosystem II have a rapid turnover in light, their steady state levels appear to decline whenever stress effects either excessively enhance deleterious oxidative conditions and degradation (e. g. Paraquat, low DCMU), or inhibit repair synthesis. Photoinactivation of catalase and of photosystem II represent specific and widely occurring early symptoms of incipient photodamage indicating stress conditions where the repair capacity is not sufficient. During prolonged exposures, e. g. to NaCl and CuSO₄, chlorophyll was bleached in light and the rate of its photodegradation increased in proportion as the catalase level had declined. The results suggest that the enhanced susceptibility of leaf tissues to photooxidative damage which is widely observed in stressed plants is related to the early loss of catalase.